AuthenticationHandler which fails to authenticate a user purporting to be one
of the blocked usernames, and blindly authenticates all other users.

Note that RejectUsersAuthenticationHandler throws an exception when the user
is found in the map. This is done to indicate that this is an extreme case
and any AuthenticationManager checking the RejectUsersAuthenticationHandler
should not continue checking other Authentication Handlers on the failure of
RejectUsersAuthenticationHandler to authenticate someone.